Fish Sex Chromosomes In general, no sex chromosomes can be identified in fish spp. A study has been started to identify sex chromosomes in Sphaerichtys osphromenoides which has only 16 chromosomes, medium size and biarmed. Thus far only females have been studied. As they are import fishes from Malaya, it is difficult to get specimens. A study in sex reversal in Xiphophorus sp and Beta splendeus which has been reported to show sex reversal (female male) showed that this is not true. In five generations we have only found late maturing males that look like females until late age when they showed their male morphology. Sex hormone treatments did not induce sex reversal. Only sterile fishes were produced. In Tilapia sp, interspecific crosses produce up to 100% male progeny, as published by others. But they never follow up the progeny from the embryonic stage. All were raised and studied in pond cultures. An experiment is planned to study fertilized eggs with sperm in vivo in containers and follow up the embryos to find out why only females survive. Another plan is to study intergeneric crosses of sunfish, which have been claimed to produce mainly female progeny. Collaborative studies are being carried out with Prof. Dr. S. Koref-Santibanex and Dr. H. J. Paepke on the chromosomes of five macropodus spp and Osphronemus gorami. Human Cytogenetics A collaborative study has been started with cytogeneticists of the Ernst Moritz Arendt University in Greifswald, East Germany on chromosomes of chorionic villi and aminocentesis cells as well as a clinical immunogenetic study on plasmacytes of human bone marrow cells with Prof. H. Storch, Karl Marx University Department of Clinical Immunology, Leipzig, East Germany.